The Action to Control Cardiovascular Risk in Diabetes (ACCORD) lipid trial aims to test whether a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin) plus a fibrate is more efficacious in reducing cardiovascular events than a statin plus placebo in patients with type 2 diabetes mellitus with defined glycemic control. This is a blinded component in a 5,518-patient subset of the ACCORD cohort. These participants were randomized to either be (1) treated with simvastatin (titrated to 40 mg/day if necessary to achieve a goal low-density lipoprotein [LDL] cholesterol level of <2.59 mmol/L [100 mg/dL]) plus placebo or (2) treated to the same goal LDL cholesterol level with the statin plus active fenofibrate 160 mg/day or its bioequivalent (or 54 mg/day if the estimated glomerular filtration rate ranges from 30 to <50 mL/min per 1.73 m2). Setting an upper limit of LDL cholesterol qualifying for randomization excluded patients who would not likely achieve the LDL cholesterol goal. Recruitment for ACCORD began in January 2001, and follow-up is scheduled to end in June 2009. Since recruitment began, several clinical trials and consensus statements have been published that led to changes in the details of the lipid treatment algorithm and protocol. This report describes the design of the lipid protocol and modifications to the protocol during the course of the study in response to and in anticipation of these developments. The current protocol is designed to provide an ethically justifiable test of combined statin plus fibrate treatment consistent with the highest level of safety and lipid treatment standards of care.